Novel hierarchical α structure enhanced strength-ductility synergy in metastable β titanium alloy

Heterogeneous structures and hierarchical structures are effective methods for overcoming strength-ductility trade-off in metals. In current study, a heterogeneous β structure, containing heterogeneity of grain size and defects (including dislocations and low-angle grain boundaries), was obtained through hot rolling and partial recrystallization processes in metastable β titanium alloy Ti-7Mo-3Nb-3Cr-3Al. On this basis, a novel hierarchical α structure was constructed through a simple aging process. The hierarchical structure couples the heterogeneous β lamella structure with a combination of small and large α phase, including alternately distributed micron-sized phases (α_WGBs, α_l) and submicron-sized α_s phases in equiaxed grain regions, and submicron-sized phases (α_l, α_p) and nanosized α_s phases in deformed grain regions. An excellent strength-ductility synergy was achieved in the designed hierarchical α structure, with a yield strength of 1360 MPa, tensile strength of 1430 MPa, and an elongation of 8.1 % at room temperature. The hierarchical α structure facilitates strain distribution and transfer during deformation, and could deform compatibly with the β matrix. Simultaneously, the grain boundary Widmanstätten α_WGBs phase reinforces grain boundary regions prone to failure, ensuring the alloy retains plasticity while enhancing strength. Back stress strengthening has been proven to be the most significant factor that enhance strength. This study provides a new simple approach for constructing hierarchical structure in metastable β titanium alloys, offering meaningful insights into achieving strength-ductility synergy.